FS7: Sensitivity in HD vs 4k?

moldcad

moldcad

Veteran
DISCLAIMER:


Due to my sickness and disability, I vane't been shooting for a couple of years now; likewise I was much less active reading and posting to the forum - hence some of my questions may sound amateurish now, with the progress of technology in the meantime.

Here is one of them:


With the FS7 sensor, do we get line-doubling and thus sensitivity boost over full 4k resulution when shooting in HD (1080p) mode?


Piotr
 
To light - sure enough, but this sensitivity as translating to the HD picture brightness when compared to 4k should be higher, right? Unless there is no "line doubling" with FS7 sensor...
 
The FS7 doesn't do line doubling or binning for HD resolution under 60fps. Instead, when it downscales it actively uses the entire sensor, providing sharper more naturally detailed and crisp results than other 1080p cameras. This is particularly noticeable when shooting fine detail objects like trees in the distance in winter.
 
moldcad said:
To light - sure enough, but this sensitivity as translating to the HD picture brightness when compared to 4k should be higher, right? Unless there is no "line doubling" with FS7 sensor...
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Its light that effects picture brightness. I'm not sure what you mean by line doubling. Are you meaning Line Skipping where the sensor skips lines to downscale to HD? The FS7 reads the full sensor up to 60fps then does some scaling/binning conversion to get the faster frame rates.

HD and 4K pictures are the same brightness.
 
FrameFarmMedia said:
Its light that effects picture brightness. I'm not sure what you mean by line doubling. Are you meaning Line Skipping where the sensor skips lines to downscale to HD? The FS7 reads the full sensor up to 60fps then does some scaling/binning conversion to get the faster frame rates.

HD and 4K pictures are the same brightness.
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He's referring to the idea that you can take a dark 4k image, and increase brightness while avoiding noise by adding adjacent photosites luma values together when downscaling. This is an effective way to increase brightness, but you lose some dynamic range, and you also lose signficant detail and resolution.
 
SNR in the shadow area is higher in HD mode due to downsampling, theoretically each time you scale to half of the original resolution, you gain 0.5 stop of shadow DR. so scaling from 8MP to 2MP gets a theoretical 1 stop shadow DR boost, but practically it usually less than that.

DXOMark has print/screen DR which takes into account this kind of downsampling, that's how 36MP D800 gets its 14.4 stops of DR.
 
Viddovation said:
He's referring to the idea that you can take a dark 4k image, and increase brightness while avoiding noise by adding adjacent photosites luma values together when downscaling. This is an effective way to increase brightness, but you lose some dynamic range, and you also lose signficant detail and resolution.
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I did not know that.
 
FrameFarmMedia said:
I did not know that.
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For a really simple way of looking at this, let's think of it in pure 8bit, and only in video terms, not raw sensor terms (this is over simplified, but gets the idea across)

A UHD frame is 3840x2160, an HD frame 1920x1080, UHD having 4 times as many RGB pixels as HD. One of the simplest ways of downscaling is to take blocks of four pixels, add together the luma values for each color channel, then divide by 4. This gets you 1920x1080, at a similar brightness to the original, but will not be as sharp or detailed as a downscale that takes edge detection and other factors into account.

But do the same process again - blocks of four pixels, add the luma values together... but don't divide by 4. You image just got brighter, but because each new brighter pixel is from 4 darker pixels, noise is lessened. However, you risk brighter values being clipped. For a more cautious version of this, add four darker pixels together, then divide only by two. But again, these methods suffer a loss of natural detail and sharpness.

Keep in mind the above is all grossly simplified theory.

The FS7 is actually very crisp at 1080, because they use the extra photosites of the 4k sensor to downsample intelligently, giving natural looking sharp edges where you'd expect them. The only pure 1080 camera that can match and exceed it that I've ever used is the Sony F35, which actually has more photosites than the FS7 does.
 
Viddovation said:
The only pure 1080 camera that can match and exceed it that I've ever used is the Sony F35, which actually has more photosites than the FS7 does.
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The newest versions of the debayering algorithm on the Amira in SUP1.0, (and maybe the Alexa in SUP11), allow you to adjust how much luma resolution gets pulled out of the 2.8K active sensor area during debayering and used in the downsample.

There is also a "Sharpness" setting now which adds very little amounts of contrast-based haloing, ala most video cameras. Although it's very well "curated" by the Arri engineers, I see little reason to destruct the image in that way. But it's there.

You can get a VERY crispy 1080 image out of the Amira (easily surpassing the 5/55), if you wanted to, via the "Detail" parameter, which is the debayering adjustment. I played around with it on mine quite a bit. A +2 or +3 on the Amira's "Detail" setting would probably match what I've seen from the F35. Anything above +2 or +3 (it goes to +5) gets into "I've never seen that before and it's distracting" territory.
 
Nate Weaver said:
The newest versions of the debayering algorithm on the Amira in SUP1.0, (and maybe the Alexa in SUP11), allow you to adjust how much luma resolution gets pulled out of the 2.8K active sensor area during debayering and used in the downsample.

There is also a "Sharpness" setting now which adds very little amounts of contrast-based haloing, ala most video cameras. Although it's very well "curated" by the Arri engineers, I see little reason to destruct the image in that way. But it's there.

You can get a VERY crispy 1080 image out of the Amira (easily surpassing the 5/55), if you wanted to, via the "Detail" parameter, which is the debayering adjustment. I played around with it on mine quite a bit. A +2 or +3 on the Amira's "Detail" setting would probably match what I've seen from the F35. Anything above +2 or +3 (it goes to +5) gets into "I've never seen that before and it's distracting" territory.
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I've never used the Amira personally, but I don't doubt Nate's appraisal. Of course, an Amira will cost you significantly more than an F35 or FS7, but that's neither here nor there.
 
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